The invention relates to a method for continuously treating bottles by cold plasma, in particular plastic bottles for containing liquids, in particular food or pharmaceutical liquids. It also relates to devices for implementing this method.
In the context of the present invention, “cold” plasma means a plasma in which only the free electrons in the gas are raised to a high average energy level by the electrical excitation, while the molecules and atoms of the gas preserve an average thermal energy virtually corresponding to the ambient energy.
The sanitized packaging of liquids in plastic bottles is an expanding branch of the food packaging industry. It serves to lengthen the shelf life and/or to improve the microbiological safety. It is intended:                on the one hand, for mineral waters that are liable to contamination by pathogenic germs, and        on the other hand, for longlife products sterilized at ultra-high temperature (UHT), to avoid reintroducing germs liable to make the products unfit for consumption (milk, soups, fruit juices).        
Furthermore, for packaging some of these products, a need exists to increase the impermeability of the bottle in order to slow down the transfers of gaseous or volatile species to and from the exterior, in particular to prevent the loss of CO2 in carbonated beverages and beer, the penetration of oxygen and/or the migration of flavors.
These sterilization and optionally impermeabilization operations must be integrated in the bottling line which extends from the molding of the bottles to the filling of said bottles.
Thus, the following operations are carried out in succession in a bottling unit:                molding of the bottles by extrusion blow molding;        optional production of a diffusion barrier, when the latter does not directly result from a multilayer including a barrier polymer;        sterilization of the finished bottle;        filling with the previously sanitized liquid;        and plugging after sterilization of the plug itself.        
In this industry, boosting production and cutting costs are a crucial concern. The succession of abovementioned operations derives, for each, from a particular technology on a dedicated machine, and implies transfers between several stations of the production line. Attempts are therefore made to reduce the duration of each step, by adjusting or by changing the technology, and to minimize the number of transfers between various stations of the line.
Conventionally, on existing bottling lines, sterilization takes place by means of oxidizing chemical germicidal liquids, such as hydrogen peroxide, peracetic acid, ozonated water, etc. The bottle is dipped or internally sprayed, optionally heated, rinsed and dried before being filled. The method is effective, but it generates liquid effluents of which the cost of treatment is added to that of the process. Moreover, in general, the management of water circuits always incurs a risk of development of inadvertent or unavoidable microbial contamination, which the companies in the sector would like to eliminate.
For the other types of container for liquid food products, such as brick packs made from cardboard/aluminum/polymer multilayers, sterilization is carried out by ultraviolet radiation, particularly in pulsed mode, possibly associated with the application of an oxidizing germicidal liquid. In the case of the combination of ultraviolet radiation with a germicidal liquid, a synergistic effect is obtained and the sterilization may be very rapid. This method, which is ideal for treating the inside aluminum surfaces of these brick packs, is nevertheless too aggressive to be applied to bottles. Moreover, the use of UV lamps has the drawback that their radiation is directional, emitted in a clearly defined and limited solid angle. Before reaching the germs to be deactivated, it is therefore subject to shadow effects due to the geometry of the container treated. This method is therefore unsuitable for the bottle geometry.
It is known that electrical discharge plasmas maintained in certain gases at reduced pressure have a deactivating effect on microorganisms. Plasma sterilizations have been considered for food containers. Thus, document EP-1 068 032 considers the possibility of reducing the microbial contamination on the inside wall of the bottle by means of an oxygen microwave plasma excited in situ (without other details). However, it is stated that the efficiency is inadequate to do without a combination with a liquid stage in a second step. No plasma action mechanism is described.
As to the impermeabilization of the bottles, various solutions are proposed.
In the present application and according to the present invention, equal use is made of the terms “impermeabilization” or “deposition of a diffusion barrier layer” to designate the operation consisting in depositing, on a surface of the bottle, a layer for limiting the diffusion of gaseous molecules from outside the bottle to the interior thereof, and from inside the bottle to the exterior thereof.
Solutions based on a multilayer coextrusion incur risks of delamination, and are costly. Resin coatings are ineffective and raise recycling problems. In both cases, the polymer barrier remains in contact with the liquid and may interact with it, thereby causing transfers of chemical contaminants.
Another solution consists in producing barrier material layers on the polymer surface of the bottles by reaction with a chemical vapor excited by a plasma (method called plasma-enhanced chemical vapor deposition or PECVD). The principles of this technique are described below.
Firstly, the electromagnetic excitation energy, which may be continuous, optionally pulsed, or alternating in a frequency range possibly extending to microwaves, is absorbed in the gas to maintain a plasma state therein.
More precisely, the electric field strongly accelerates the free electrons present in the plasma. During their very rapid movement in the electric field, the electrons constantly undergo very frequent elastic collisions with the gas molecules.
Thus, they assume a statistical kinetic energy distribution similar to the conventional thermal agitation of the particles of a gas, but forced by the electrical excitation. The average kinetic energy acquired by the electrons by this mechanism is extremely high. It could be equivalent to a temperature for the electrons (thus by treating the average energy as kT, where k is the Boltzmann constant and T the absolute temperature in kelvin) of about several tens of thousands of kelvin.
However, the molecules and atoms of the initial gas do not directly receive the energy from the electric field and therefore preserve their statistical motion of natural thermal agitation. If the gas is initially cold, it remains so even when excited to pass into the plasma state. This is therefore referred to as “cold plasma”. This particular state of a gas medium is generally engendered under reduced pressure. If the pressure is too close to atmospheric pressure, the elastic collisions of the electrons with the heavy gas particles, atoms and molecules, become so frequent that these particles themselves ultimately receive a high energy via said elastic collisions and their temperature may rise considerably. The plasma then deviates from the state that is advantageous for PECVD.
In the cold plasma, a large number of electrons have sufficient energy to cause inelastic collisions with the gas molecules, with the effect of an excitation, an ionization or a dissociation.
Ionization corresponds to the stripping of an electron from an atom or a molecule to create an electron-ion pair. This continuous production of new charged particles compensates for the losses of such particles by recombination in the volume or at the wall, and serves to maintain the plasma in steady state condition.
Dissociation of the initial gas molecules produces smaller fragments, atoms and radicals, comprising pending open chemical bonds which make these gas species extremely reactive, either with a solid surface, or with each other in the gas phase. In particular, the radicals formed from chemical molecules initially introduced into the gas, will be capable of reacting with the substrate surface to culminate in the incorporation of all or part of their constituent atoms in the lattice of a solid material of which a thin layer will thus grow progressively on the substrate surface. The reactivity of the radicals with the surface is so high that this incorporation and growth process does not require the surface to be raised to a temperature above ambient temperature to activate the reactions.
The excitation of the gas species, conferred by the inelastic electron collisions, is equivalent to raising these species to one of their energy levels quantified as electronic or vibrational, higher than the fundamental level. The order of magnitude of these energies is several electron-volts. To obtain such levels by a hypothetical heating of a gas, the temperature of this gas should therefore be several tens of thousands of kelvin or more. In a cold plasma, only a small fraction of the total number of heavy particles are raised to such energy levels, while the others remain close to their fundamental state, corresponding to ambient temperature.
This is referred to as nonthermal energy excitations. This energy carried by certain molecules, atoms, radicals or ions of the plasma can then be liberated at the level of the substrate surface when said species reach it. Its main advantage will be to assist the migration and rearrangement of the atoms during their incorporation in the material of the solid film. This makes it possible to deposit a high grade material, that is having good connectivity and a minimum of vacancies in the atomic lattice, and free of granular or columnar microstructures; this occurs without necessarily having to heat the substrate to a temperature significantly higher than ambient temperature, for example of 200 to 400° C., which is known to improve the quality but cannot be applied in the case of a polymeric substrate.
Another form of nonthermal energy which can be conveyed to the surface of a substrate in contact with a cold plasma, is that originating from the impact of ions accelerated by a deliberately applied potential difference between the plasma and the substrate, in a manner known per se.
A PECVD process for depositing barrier films on polymer models for food liquids must, in addition to an appropriate quality of the material, serve to guarantee a high deposition rate so that the technique is compatible with the production rates in this industry, and economically viable. A deposition rate of about 100 to 1000 nm/minute is generally suitable for depositing a layer having a thickness of a few tens to one hundred nm.
A high deposition rate implies the creation of a high concentration of precursor radicals capable of effectively condensing and reacting on the solid surface of the substrate and participating in the growth of the barrier layer. For this purpose, it is in particular necessary for the electron density of the plasma to be high, so that a sufficient number of electrons having the requisite energy are available to cause the inelastic collisions culminating in the formation of such precursor radicals.
To simultaneously maintain the quality of the layer material, it is clear that the input of nonthermal energy by excited species must be proportional to the average flux of atoms condensing on the surface to form the solid film. In fact, the higher the number of atoms incorporated per unit of time, the denser and also higher the nonthermal energy flux required to rearrange them by forming a regular atomic lattice.
The minimum nonthermal energy flux that may be required for deposition on the surface of the growing film to obtain good quality, depends on the material considered and on the chemistry of the gas phase. Moreover, this flux is also related to the pressure of the processed gas. The higher the pressure, the more the radicals tend to react prematurely in the gas phase before being individually positioned on the substrate surface. The reactions between radicals in the homogenous gas phase culminate in the formation of bonded atom clusters of larger size. When such an atom cluster reaches the surface, it tends to be incorporated while preserving its pre-existing atomic arrangement, by establishing bonds with the matrix and with neighboring clusters. This produces a less uniform and denser structure than that which would correspond to an optimal individual arrangement of each of the atoms in the lattice of the material constituting the thin film. To avoid this, added nonthermal energy must be available to dissociate the clusters reaching the surface so that the component atoms can then enter into an optimal lattice arrangement.
In practice, the various steps of the PECVD process described above (or more generally any cold plasma surface treatment process, in particular a sterilization treatment), must also be carried out by controlling the spatial distribution of the mechanisms. This is because the objects to be treated generally have a non-negligible size and the result of the treatment must be uniform throughout the surface of the substrate concerned. The effects of the treatment must not be exacerbated at certain locations, with potential damage to the substrate, and insufficient or non-existent elsewhere. For example, a deposited thin layer thickness must not vary by more than a few percent between any two points of the surface of a part to be coated, with a material quality that remains substantially the same everywhere.
In fact, the active species involved for example in a PECVD process, depositing radicals and particles carrying nonthermal excitation, correspond to transient states and have a short lifetime. More precisely, their mean path in the gas phase between their creation and their deexcitation and/or recombination (after which they have lost their advantageous properties for the method) have the same order of magnitude as the characteristic dimensions of a bottle. The plasma zone where the active species are created following the inelastic electron collisions must therefore be spread and fairly closely match the shape of the bottle surface. Moreover, the absorption of the electromagnetic power to maintain the plasma and to promote the inelastic electron collisions producing the active species, must be relatively uniform in this distributed plasma zone. In this way, the plasma treatment can be sufficiently rapid and complete.
However, it is a complex technical problem to supply electromagnetic power and to make it absorbed substantially uniformly to maintain the plasma in an arbitrary region of the space distributed in the vicinity of the object to be treated. This is because the power transfer is governed by the laws of electromagnetism, and also in a medium that is highly absorbent by definition. In particular, if one attempts to propagate progressive waves, they are rapidly damped due to the absorption along their propagation direction, hence a natural nonuniformity of the plasma thereby created.
It is not sufficient to control the plasma distribution to obtain a uniform treatment. The active species created must be effectively transportable to the surface, along a similar path (in the sense of its length and of the ambience crossed) for all of them. This transport is governed by the diffusion and dynamic conditions of the gas stream in the treatment device. For example, it is possible for a nonhomogenous boundary layer to be formed in the vicinity of the substrate surface by radical depletion. In fact, the resistivity of these radicals is very high, so that their consumption at the surface is much faster than their transport in the gas phase. The limitation of the deposition rate by transport in the gas phase generally leads to a nonuniform distribution imposed by the dynamic of the gas stream when a gas flow is maintained to continuously replenish the vapor of the chemical precursor consumed, as is generally the case in an industrial PECVD process.
All these problems are aggravated in the case of a bottle for beverages, which is an object having an awkward shape, having a high degree of geometric symmetry and a substantial extension (capacity up to 2 liters), whereas in the usual industrial cases, PECVD is applied to planar substrates of circular or rectangular shape. This requires the solution of highly complex problems of engineering of the plasma production device and of the deposition reactor.
Some authors (see for example documents U.S. Pat. Nos. 6,627,163, 5,904,866, US2005/0019209) have nonetheless come to a standstill on these aspects.
The technical solutions really available today for producing barriers on plastic bottles by PECVD have been forced to integrate specific technical options to contend with the abovementioned difficulties.
Thus SIDEL (commercial process known by the name “ACTIS”) uses a microwave plasma excitation. The problem of the distribution and distributed absorption of the microwaves was circumvented, so to speak, by placing the entire bottle in a resonant cavity supplied at the frequency of 2.45 GHz. The bottle is placed in a dielectric chamber having a slightly larger diameter, itself placed in the conductive structure of the resonant cavity. The deposition method requires a vacuum of about 0.1 mbar in the bottle, implying a pumping installation of sufficient size. The chamber surrounding the bottle is also pumped, but to a lower vacuum, to avoid the contraction and crushing of the bottle, and also to prevent the undesirable ignition of a second plasma at the exterior.
Moreover, the deposition is carried out in static conditions, that is, the gas mixture comprising the chemical precursor is previously introduced under the pressure specified in the bottle, which is then isolated from the exterior. The plasma is then established to dissociate the chemical precursor vapor and to deposit the barrier layer. Due to the surface consumption of the precursor, a concentration gradient of active species between the gas phase and the surface is established. However, in static conditions, this gradient is the same at every point of the surface. Moreover, since the layer is very thin and the deposition step is short, the chemical precursor is not generally consumed in a high proportion and the average concentration in the gas phase does not decrease sharply inside the bottle over the deposition time.
The resonant cavity excitation mode nevertheless has certain drawbacks.
In a resonant cavity, only a series of discreet electromagnetic field distribution modes can exist, modes specific to the geometry of the cavity and therefore fixed once and for all. These eigenmodes of the cavity each correspond to a given distribution of the microwave field intensity in the cavity, and hence the distribution of the plasma density which is maintained by absorption of the energy of this microwave field. The inventors have found that in a cavity having a certain size, an eigenmode can be maintained in which the microwave field intensity distribution does not vary too much axially within a volume in which a bottle having a capacity of 600 ml may be enclosed. On the other hand, for higher cavity sizes, no such mode exists for which the field is sufficiently uniform axially to treat bottles having a larger capacity. In particular, commercial bottles having a capacity of 1.0 to 2.0 liters cannot be treated by this technique.
Another drawback of the “SIDEL ACTIS” microwave plasma device, which is also inherent in the excitation by resonant cavity, resides in the slight possibilities offered by the deposition of a controlled nonthermal energy on the inside surface of the bottle to promote the quality of the deposit. In fact, the microwave field does not have a pronounced maximum intensity in the vicinity of the bottle surface. In consequence, the creation of high internal nonthermal energy species under the effect of the inelastic electron collisions is not particularly promoted in this zone.
Nor is it possible, in this arrangement, to magnify and control the bombardment of the inside surface of the bottle by the plasma ions. The bottle is made from a dielectric material and there is no obvious means of charging it negatively in a distributable and adjustable manner. For example, it is not possible to apply a radiofrequency bias uniformly to this surface by means of a conductive electrode surrounding the bottle, because in this case, the microwaves can no longer pass through the wall of the bottle to maintain a plasma inside it.
Another solution could be to inject fast electrons produced by an electron gun toward the surface, as proposed by certain authors, but this alternative is neither simple nor inexpensive, and its practicability inside the bottle remains hypothetical.
This inadequacy in terms of nonthermal energy input on the bottle surface limits the choice of barrier materials of acceptable quality which can be deposited by this technique. It must in fact be limited to the deposit precursor chemistries which are known to be capable of providing a material of sufficient quality even under these conditions. This is the case for the deposition of hydrogenated amorphous carbon from the monomer acetylene. The latter has the drawback of a pronounced yellow color which makes it incompatible with certain applications such as containers for drinking water. It is also possible to deposit from organosilicate precursors layers still having a pronounced organic character. On the other hand, there is no commercial method based on this concept that would serve to deposit layers of inorganic silicon alloys of the type SiOxNyCzHt which could be useful for optimizing the functionalities of barrier coatings.
Another solution is proposed by SIG Corpoplast with its “Plasmax” process. In this process, the device for applying microwaves to create a plasma in contact and close to the bottle surface, consists of a conductive chamber relatively closely surrounding the bottle, inside which the microwaves are injected by an antenna supplied with power via a waveguide. This structure is not a resonant cavity and does not have the geometry thereof. It is rather a hybrid structure from the electromagnetic standpoint, partially propagated and partially stationary. The microwave field can be expected to have wide inhomogeneities in the dimensions of the structure, with, on the one hand, standing wave intensity nodes and antinodes, and on the other hand, a rapid average axial decrease of intensity due to the property of propagating in an absorbent medium.
To succeed despite this in producing a relatively homogenous deposit on the whole inside surface of the bottle, the operating conditions are such that the deposition rate is not limited by the intensity of the plasma. More precisely, the injected microwave power is selected to be sufficiently high so that, at any point of the surface, the process of creating depositing radicals by dissociation of the precursor molecules reaches its saturation value with regard to the power. Thus, the deposition rate is imposed at every point by the precursor concentration and not by the microwave field intensity.
However, these conditions cannot be used continuously because due to the deliberately high intensity of the microwave field and of the plasma, the bottle material would rapidly suffer serious damage. To avoid this, a pulsed microwave power supply is used, the pulse duration and the repetition rate being adjusted so that the excess energy deposited, by finally being converted to heat, can be removed between two pulses.
The pulse power supply also serves, in a manner per se, to improve the uniformity of deposition because the gas phase in the vicinity of the bottle surface, depleted of active radicals during a deposition pulse, can be re-enriched between two consecutive pulses.
On the other hand, the control of the deposited nonthermal energy is highly imperfect in this arrangement. In fact, if the flow of depositing radicals is relatively uniform under these conditions, the same cannot be said of the nonthermal energy, which follows the spatial variations in intensity of the microwave field and of the plasma. In the development of the method, the pulse regime is adjusted so that no unacceptable damage can appear on the portions of the bottle surface which experience the lowest energy flux. This does not guarantee that the portions subject to the lowest energy are under optimal conditions for the compromise between radical flux and flux of excited nonthermal species, that is, deposition rates/layer quality. Thus it may be necessary to decrease the precursor concentration to lower the deposition rate. This limitation is undesirable because the potential users of this technology still demand a substantial increase in the treatment rate, which should typically rise from 10,000 to 50,000 bottles/hour.
In document WO2006010509 (KRONES) mention is made of such a combined treatment, but no details are provided for its implementation.
Thus a real need exists for a method for depositing impermeabilization layers while decreasing or eliminating the inadequacies of the current solutions, and/or a method for sterilizing, said method being intended to be integrated in a conventional bottling process, and not generating any aqueous effluents, not using germicidal chemical compounds, and implemented with a limited number of transfer steps.